Interpretive Summary: Avian influenza virus (AIV) and Newcastle disease virus (NDV) are two of the most economically important viruses affecting poultry. Because these viruses cause similar clinical signs in infected birds, co-infection of poultry with these viruses is difficult to recognize. Co-infections also may affect the severity of clinical signs, virus shedding and virus transmission. In this study we infected chickens and turkeys with a lentogenic NDV and a low pathogenic (LP)AIV by giving the viruses simultaneously or sequentially. No clinical signs were observed in any of the chickens, co-infected or not, but virus shedding was affected, with co-infected birds shedding less virus at 2 and 3 days post inoculation and shedding more virus in subsequent time points than birds infected with the single viruses. Turkeys inoculated with the LPAIV, co-infected or not, presented similar mild clinical signs, and LPAIV clearly affected NDV virus shedding. In conclusion, previous or simultaneous infection of lentogenic NDV and LPAIV affected the replication dynamics of these viruses in poultry but did not have an effect on clinical signs.

Technical Abstract:
Avian influenza virus (AIV) and Newcastle disease virus (NDV) are two of the most important viruses affecting poultry worldwide. Exposure to lentogenic NDV, either from live vaccines or field strains, is nearly unavoidable for poultry, and co-infections with low pathogenic (LP) AIV are expected to occur in LPAIV outbreaks. Because of the similarity in clinical presentation, co-infection of poultry with these viruses is not easily detected and it’s not know if co-infection affects clinical signs, virus shedding and virus transmission. In this study we infected chickens and turkeys with a lentogenic NDV vaccine strain (LaSota), and a LPAIV (subtype H7N2) by giving the viruses simultaneously or sequentially. Pathogenesis, duration and titer of virus shedding and seroconversion to both viruses were evaluated. No clinical signs were observed in chickens co-infected with the lentogenic NDV and the LPAIV or in chickens infected with the two viruses given separately. However, the pattern of virus shedding was different, with co-infected birds shedding less amount of virus at 2 and 3 days post inoculation and shedding more virus in subsequent time points than birds infected with the single viruses. All turkeys inoculated with the LPAIV, co-infected or not, presented similar mild clinical signs. Like chickens, co-infected turkeys had altered patterns of virus shedding which was especially evident in the group that received the LPAIV followed by NDV. In conclusion, previous or simultaneous infection of lentogenic NDV and LPAIV affected the replication dynamics of these viruses in poultry but did not have an effect on clinical signs.